A relatively traditional wiper system for the window of an automotive vehicle is depicted in FIG. 1 herein. That wiper system is depicted as including a depressed park capability and an intermittent, or delay, feature. That system includes a two-speed wiper motor 110, a cammed Run switch 112, a cammed Park switch 114, conventional drive and control electronics 116, five ganged high-current wiper mode switches, generally represented as 118 and a potentiometer 120. The potentiometer is used to adjust the delay interval for intermittent operation. The cammed Run switch 112 carries the current of wiper motor 110 while the wiper, or wipers, represented by reference numeral 122 is sweeping across the windshield. The cammed Park switch 114 is used to reverse the direction of rotation of wiper motor 110 which in turn brings the wiper 122 to the "Depressed Park" position. The forward and return sweeping motion of the wiper 122 across a windshield is performed in the linkage between it and the wiper motor 110 such that the motor is normally only rotated in a single direction, but is reversed to move the wiper 122 into the "Depressed Park" position.
More recently, there has been developed a wiper control system of the type described in U.S. Pat. No. 4,578,591 for Control Circuit For Automotive Accessory System filed by Floyd et al, issued Mar. 26, 1986 and having the same assignee as the present application. The wiper described therein was in some respects the same as that described in the preceding paragraph; however, it further described a system in which a reduced number of low current input switches might be used in conjunction with a microcomputer based controller for directing the control of the windshield wipers. The input control signals to the microcomputer and the resulting drive signals to the wiper motor were conducted via a serial multiplex link and respective remote multiplex (REMUX) controllers or slave units. If the wipers are to be operated in an "intermittent" mode, the length of operator dwell on the appropriate input switch serves to store a count in the central controller which is representative of the length, or duration, of that dwell. An appropriate program then applies that dwell each time the wiper completes a wipe cycle. However, the actual operation of the wipers was dictated by applying battery voltage to the appropriate poles of the motor to effect rotation of the motor in the appropriate direction and in one of the two (low or high) nominal speeds.
Such arrangements, however, have certain limitations, one of which includes the electrical and acoustical noise produced while the cammed switches are carrying current and switching states. Another problem may reside in the relative inability of those prior art systems to provide and/or maintain a desired speed profile for the wipers. In this latter regard, apart from various nonlinearities introduced by the mechanical linkage between the motor and the wipers, the speed of the wipers has typically been viewed as being constant as a result of applying a nominally-constant battery potential to the drive motor. In reality, however, variations occur in the level of the voltage applied by the battery, which variations result in corresponding changes in the wiper speed. Additionally, changes in the torque loading on the motor, as occasioned by variations in the friction in the mechanical system and at the interface between the wiper and a wet or dry windshield surface, will also affect the speed of the wipers. While some such uncontrolled variation in wiper speed may be tolerable, it is generally desirable to maintain a selected speed as accurately as possible. Also, it is often desirable to be able to select between two or more of such accurately maintained speeds.
Another type of variation in wiper speed is occasioned by differences in sweep, or stroke, speed between the forward sweep and the return sweep. It is generally desirable that these two sweeps, which comprise a wiper cycle, each be at about the same speed. However, certain characteristics in the mechanics of the wiper drive system can cause differences between the two sweep speeds which may be viewed as excessive.
A further concern with respect to the dynamics of wiper operation resides in the relatively abrupt manner in which the wiper reverses directions. Typically, the wiper will arrive at an inflection point, or point of reversal, with little or no prior deceleration. Such operation places a strain on the mechanical linkage of the drive train and possibly also on the wiper blade structure.
One recent wiper system is disclosed in U.S. Pat. No. 4,544,870 to Kearns et al for Intermittent Windshield Wiper Control System With Improved Motor Speed Control. In that patent there are disclosed several wiper control systems which seek to control wiper speed. Most, or all, of those systems rely upon the use of a transistor operating as a linear amplifier for regulating wiper motor speed. That system also results in the relatively large dissipation of energy in the form of heat. Moreover, there does not appear to be any provision for automatically regulating wiper speed in the event of changes in the supply voltage.
Although there has been some discussion in the prior art of using pulse-width modulated control signals for adjusting wiper motor speed, there remains the need for a system which provides accurate and repeatable speed control using pulse-width modulation. Moreover, pulse-width modulation circuits operating at frequencies greater than about one KHz, and typically about 15-25 KHz, emit such electromagnetic radiated and conducted interference noise that filtering circuits are necessary. This noise increases as switching speed increases.
Accordingly, it is a principal object of the present invention to provide an improved control for a window wiper system which does not possess the limitations of the prior art, but which is repeatably accurate in operation. Included in this object is the provision of a wiper control system which accurately and automatically maintains a predetermined speed or speeds of the wipers.
It is a further object to provide a wiper control system which facilitates the profiling of desired wiper sweep speed characteristics. Included within this object is the provision of an apparent dwell interval without requiring the wipers to be at rest for any significant period.
It is a still further object to provide a wiper control system which employs solid-state electronic components configured and operating in a manner which minimizes energy loss, either in thermal form or as electromagnetic interference.
It is an even further object to provide a wiper control system which acquires feedback information in a simple, low-cost manner without requiring extensive electronics or sensing arrangements.